Resistors

Definition and Function

A resistor is a component in an electrical circuit that limits the flow of electric current. It provides resistance, which opposes the movement of electrons through the circuit.

Resistors are used to protect sensitive components by controlling the amount of current that passes through them. Without resistors, components like LEDs or microchips could receive too much current and be damaged.

In simple terms, resistors act like a narrowing in a water pipe, reducing the flow of water (current) to a safe level.

Resistance

Resistance is the measure of how much a resistor opposes the flow of electric current. It is measured in ohms (Ω).

The resistance of a resistor depends on:

• Material: Some materials resist current more than others. Metals usually have low resistance, while insulators have very high resistance.
• Length: The longer the resistor, the higher the resistance because electrons collide more often.
• Thickness (cross-sectional area): Thicker resistors have lower resistance as there is more space for electrons to flow.

Resistance can be thought of as the "friction" that slows down the flow of electrons in a circuit.

For example, a copper wire has much lower resistance than a thin piece of nichrome wire of the same length because copper is a better conductor and the thickness and material affect resistance.

The symbol for resistance is $R$, and the unit ohm is represented by the Greek letter omega (Ω).

The relationship between potential difference (voltage), current, and resistance is given by Ohm’s Law (covered in another topic). However, understanding resistance itself is key to controlling current in circuits.

Example: If a resistor has a resistance of 10 Ω, it means it opposes the current flow with 10 ohms of resistance.

Types of Resistors

There are different types of resistors used in circuits:

• Fixed Resistors: These have a set resistance value that does not change. They are the most common type and come in various sizes and power ratings.
• Variable Resistors (Potentiometers): These allow the resistance to be adjusted manually. They are useful for controlling devices like volume controls or light dimmers.

Note: Special resistors such as thermistors and light-dependent resistors (LDRs) change resistance with temperature or light but are covered in other topics.

Resistors in Circuits

When resistors are added to a circuit, they affect the current and voltage:

• Current: Adding a resistor reduces the current flowing through the circuit because it opposes the flow of electrons.
• Voltage: A resistor causes a drop in potential difference (voltage) across it, proportional to the current and resistance.

The total resistance in a circuit depends on how resistors are connected:

• In series: The total resistance is the sum of all resistors. This increases the overall resistance and reduces current.
• In parallel: The total resistance is less than the smallest individual resistor because the current has multiple paths to flow.

For detailed calculations and behaviour of resistors in series and parallel circuits, see the "Series & Parallel Circuits" topic.

Learning Example

A fixed resistor has a resistance of 5 Ω. If the current flowing through it is 2 A, the voltage across the resistor can be found using Ohm’s Law (covered elsewhere):

For instance, if $I = 2 \text{ A}$ and $R = 5 \ \Omega$, then $V = IR = 2 \times 5 = 10 \text{ V}$.

This means the resistor causes a 10 V drop in potential difference when 2 A flows through it.